1. Field of the Invention
This invention relates to coating systems for the hot section components such as blades and integral/segmented nozzle vanes of gas turbine engines, and, more particularly, to coating systems which provide the coated hot section component with improved oxidation/corrosion resistance.
2. The Prior Art
Materials used for the fabrication of gas turbine components must have both exceptional elevated temperature mechanical properties and resistance to surface degradation such as oxidation and hot corrosion at elevated temperatures.
In current gas turbines, temperatures range in excess of 2500.degree. F. and it is desired that such engines operate for prolonged periods of time without undergoing significant materials degradation.
The current high cost of quality fuels for gas turbines has made it economically attractive to use lower quality fuels or to increase the temperature of the turbine. These lower quality fuels may contain harmful alkali-sulfates which cause accelerated hot corrosion attack of the hot gas path components of gas turbines. The hot gas path components, such as vanes and blades, are generally constructed of nickel base or cobalt base superalloys. The superalloys, while possessing high strength at high temperatures, are quite prone to the accelerated corrosive effects of the hot gas path.
To prevent unacceptably rapid oxidation and corrosion rates of the hot path components, protective coatings are necessarily utilized to prolong the useful life of the components. The typical coating provides the superalloy with a surface layer characterized by increased oxidation and corrosion resistance. In the gas turbine industry, this protective layer is often formed of an aluminide which is produced by diffusion of aluminum into the surface of the hot section component to be protected, and the reaction of the aluminum with the superalloy substrate material to produce intermetallic compounds. In use, the surface of the component develops an alumina layer which acts as a barrier to prevent further oxidation of the coated component. A drawback to the use of aluminide coatings is that the coatings can be a source of fracture initiation in fatigue. Coating ductility has been found to be an important determinant in fatigue life since, at relatively low temperatures, aluminide coatings tend to crack in a brittle manner at low strains in the tensile portions of the fatigue cycle.
A second type of protective coating used for imparting oxidation/corrosion resistance to gas turbine hot section components are overlay coatings. Overlay coatings are themselves oxidation resistant and do not depend upon any reaction with of diffusion into a substrate. Typical of the overlay coatings in use today are those designated as "MCrAlY" coatings where M is nickel, cobalt, iron or mixtures thereof. A drawback to the use of MCrAlY compositions as protective coatings for gas turbine hot section components is that these coatings are conventionally applied to substrates using physical vapor deposition methods, which methods exhibit line of sight limitations. Line of sight limitation means that the material to be coated is contained within the conical angle emanating from the source (for e.g. plasma spray guns, sputter targets, etc) and coats the substrate only in the exposed areas within the conical angle spray. Such limitation results in providing incomplete coverage to integral/segmented components or complex shaped individual parts due to shadowing effects.
It is, therefore, an object of this invention to provide a metal coating composition as well as a coated article which are devoid of the above-noted disadvantages.
It is another object of this invention to produce coating compositions for use in hot, corrosive, combustion atmopsheres of the type found in gas turbines.
It is still another object of the present invention to provide coating compositions which may be applied to nickel base, cobalt base or nickel-cobalt base superalloys, and which are highly resistant to hot corrosive attack.
It is yet another object of this invention to provide high temperature metal coating compositions wherein there is increased wettability or diffusional bonding between the layers of the coating structure, resulting in reduced sites (microporosity) for thermal fatigue crack initiation and/or spallation and, hence, superior performance.